During this, she imagines a man who is the opposite of fiancé Cal in every way: kind, spontaneous, noble, selfless, and full of life.

Then Jack Dawson appears! Rose believes Jack is real because she’s desperate to break free from her mother, and from Cal. Jack convinces her to "break free to stay alive," thereby voicing her own subconscious, and making her stronger and braver.

Then, when the ship sinks, the famous door scene occurs, where Rose survives and Jack dies - because according to the theory, he was never real anyway. Rose tells him she’ll ‘never let go’, and then lets him drift into the murky waters.

The idea behind it being he could never get out anyway - but she’ll take with her the lessons of survival he taught her.

The biggest real ‘evidence’ of this theory is the fact that the bearded dude from the excavation crew says that there was never any record of Jack on the passenger list, though, many would argue it’s because he won tickets only a few minutes before the ship sailed.

The other reason it may not be true is because you see Jack interacting with lots of other people during the movie, although, as the movie is entirely from Rose’s perspective, she may just be an unreliable narrator.

But we are choosing to believe he WAS real and they were reunited in heaven and all was well and good!